In the field of this invention it is known, as shown in FIG. 2, that AGC (Automatic Gain Control) is a process used in a mobile radio receiver whereby the mobile adjusts the gain in the analogue sections of the receiver such that the signal is of the correct magnitude at the input to an ADC (Analogue to Digital Converter). If the signal is too large, then the signal will be limited or ‘clipped’, whereas if it is too small, it will be susceptible to significant signal-to-noise ratio degradation due to the quantisation process of the converter.
AGC must therefore adapt to the fluctuations in received power over time. Such fluctuations occur in mobile radio due to:                the mean pathloss between transmitter and receiver,        constructive and destructive interference between multiple transmission paths between transmitter and receiver (so-called fast-fading), and        variations in the transmitted power of the wanted and interfering signals.        
In a 3GPP TDD-CDMA radio communications system, the radio resource may be considered, as shown in FIG. 3, as divided into three orthogonal planes: namely those of frequency, time, and code. Each cell or cell-sector is assigned a specific frequency in which to operate. Within this frequency allocation, the resource is split into time frames, each of length 10 ms. Each time-frame is further sub-divided into 15 timeslots, each of length 666.67 μs. Each timeslot is divided in the code-domain into 16 channelisation or ‘spreading’ codes. The properties of these codes are arranged such as to enable extraction of the information transmitted over each code from the ‘multi-code’ composite signal by signal processing means. The system may therefore distribute select information towards targeted users by means of assigning them resource defined in the three co-ordinate resource-space of frequency, time and code.
Many AGC loop designs exist (such as the analogue-monitored signal AGC loop shown in FIG. 4 and the digital-monitored loop shown in FIG. 5), but in general the loops are designed to monitor the received signal at the ADC input, or output, and provide negative feedback to the analogue variable receiver gain section in an attempt to maintain the monitored signal at a constant target level. In general, the measured characteristic of the monitored signal is peak-voltage, peak-power, or mean power. Thus, if the measured characteristic of the monitored signal is higher than the target, the analogue gain of the receiver is lowered, whereas if the characteristic of the monitored signal is lower than the target, the gain of the receiver is increased.
However, this known approach has the disadvantage(s) that:    1) In a packet-radio system such as a TDD-CDMA system, for a particular cell frequency, the power transmitted on a timeslot is, in general, a function of the number of codes transmitted. Thus, given the timeslot-segmented nature of the TDD-CDMA system, the power transmitted in each timeslot may vary considerably as the number of codes varies. The mobile-station, although aware of its own timeslot/code allocations, is not usually aware of allocations to other users and therefore cannot predict how much power will be received in a given timeslot. This therefore presents difficulties for AGC since it is the function of AGC to adjust the receiver analogue gain in response to the received power such that signal presented to the ADC is at an appropriate level.    2) For TDD-CDMA, a further problem exists for AGC due to the Time Division Duplex nature of the system. As illustrated in FIG. 6, during the initial synchronisation phase, the mobile station must search for a specific synchronisation code transmitted by the network. At this point, the mobile station does not have any knowledge of the frame timing of the system. Due to the fact that uplink timeslots are transmitted on the same frequency as downlink timeslots (but are separated in time within the frame), without knowledge of the frame timing, the mobile station must configure itself to receive on all timeslots in search of the synchronisation code. The mobile station receiver is therefore subject to reception of uplink signals from nearby mobile stations on the same cell frequency. These uplink signals may be hundreds of times larger in power than the downlink signal that the mobile is trying to detect. As a result, any AGC loop that tries to track the received signal power over the whole radio frame will try to accommodate the large uplink signal and may consequently suppress the (relatively small) wanted downlink synchronisation signal such as to render it undetectable (i.e., it is possible that the wanted signal in timeslot 0 will occupy so little of the ADC input voltage range as to render it undetectable).
Even if the signal is detectable, the synchronisation correlation peak in timeslot 0 may be much smaller than the correlation noise peak occurring in the timeslot with highest power. This will result in a synchronisation lock failure, or a false detection (which will also eventually lead to a synchronisation failure).
Although this problem will not always exist, it is desirable to implement a receiver strategy that provides robustness under these adverse conditions, since an intermittent inability to acquire synchronisation will obviously result in a high level of user dissatisfaction. Such conditions are likely to occur in any environment where there is a high possibility of users being in close proximity to each other.
A need therefore exists for an AGC scheme and limiting receiver architecture wherein the abovementioned disadvantage(s) may be alleviated.